The herbicidal activity of 2-(4-aryloxyphenoxy)propionic acids and derivatives thereof is well known in the art. Furthermore, optical isomers are often known to exhibit enhanced herbicidal activity over the corresponding racemates. For example, U.S. Pat. No. 4,531,969 discloses that the R-enantiomers of certain 2-(4-aryloxyphenoxy)propionic acids and certain derivatives thereof are distinguished by a considerably enhanced herbicidal action compared to the racemic modifications. Since reduced quantities of herbicide are required to achieve comparable levels of control, the application of mixtures enriched in the more efficacious R-enantiomer offers both economical and environmental advantages. Various methods for obtaining high concentrations of optical isomers are known. In addition to the resolution of a racemic mixture into its optically active components which, for example, depends on the conversion to diastereomers and subsequent physical separation, individual enantiomers can be obtained by direct synthesis employing an appropriate optically active starting material. For example, optically active alkyl 2-(4-aryloxyphenoxy)propionates are conveniently prepared by reaction of either an optically active alkyl 2-halopropionate or an optically active lactate ester of an alkyl or aryl sulfonate with a 4-aryloxyphenol as depicted in Equation 1 where the leaving group is a halogen or an alkyl or aryl sulfonate. The * represents an asymmetric carbon atom. ##STR1##
Theoretically, one can obtain essentially 100 percent of the desired enantiomer by this method. In practice, however, the optical purity of the final product is largely influenced by such factors as the optical purity of the starting material and the specific reaction conditions. Typically one obtains products containing a ratio of from 70 to 90 percent of the desired enantiomer and, correspondingly, from 10 to 30 percent of the other optical isomer. Such products are then said to possess an optical purity of 40 to 80 percent, i.e., from 40 to 80 percent of the mixture is the desired enantiomer and from 20 to 60 percent is a racemic mixture.
Partial racemization over the course of a reaction is somewhat typical of nucleophilic displacement reactions which involve the breaking and making of bonds at the asymmetric carbon atom of the starting material. Although such reactions mechanistically involve inversion of configuration, minor complications such as competing reactions can significantly adulterate the optical purity of the product.
An alternative approach for preparing optically active alkyl 2-(4-aryloxyphenoxy)propionates involves the reaction of an optically active alkyl 2-(4-hydroxyphenoxy)propionate with an aryl substrate having a leaving group activated with respect to aromatic nucleophilic substitution. This reaction is illustrated in Equation 2. ##STR2## Since this reaction does not involve bond-breaking or bond-making with the asymmetric carbon atom of the optically active 2-(4-hydroxyphenoxy)propionate, very little if any racemization is expected, particularly in the presence of such relatively mild bases as the alkali metal carbonates which are typically used in such reactions. Unexpectedly, it has now been recognized that racemization during this reaction can be appreciable, especially for esters, i.e., when R is a lower alkyl group. The extent of racemization is dependent on the reaction conditions and on the type of aryl substrate employed.
Probably one of the most conventional ways to reduce the occurrence of racemization is to lower the reaction temperature. Such conventional practice, however, is not thoroughly effective and contributes to greatly extended reaction times. A more efficient and effective solution to this problem is desirable.